The present invention relates in general to communication systems, and is particularly directed to a scheme for increasing the channel capacity of a wireless local loop (WLL) network by means of a prescribed polarization diversity antenna assignment scheme among geographical cells of the WLL communication network.
As the communications industry continues to expand into what were previously remote and/or less technologically developed regions of the world, it faces the absence of an existing communication (e.g., copper wire, fiber optic link) cable infrastructure with which to connect its equipment. Because the cost and time to install such an infrastructure in these regions is prohibitive, communication service providers have turned to the use of radio-based system, known as wireless local loop (WLL) networks.
As diagrammatically illustrated in FIG. 1, in a wireless local loop, customer premises equipments are typically comprised of one or more telephones (e.g., handsets) 10, that are coupled to a wireless transceiver (e.g, radio) 11 installed within a building 13 (such as a business or home), and having an associated fixed (e.g., roof-mounted) antenna 15. As shown in FIG. 2, these customer premises equipments are located at a number of fixed sites 17 that are geographically dispersed relative to a base station 20, where radio transceiver equipment of the WLL service provider for the geographic cell of interest is located. The WLL base station 20, in turn, is coupled to rout customer calls through a public telephone switch network (PTSN), so that each WLL network customer of the cell may enjoy the same services provided by a wireline-based telco subscriber.
FIG. 3 diagrammatically illustrates a typical layout of a large area WLL multicell structure. The darkened cells have the same channel frequency allocations in accordance with a frequency reuse factor of four. Cells B1-B6 are cells reusing the same frequencies as cell A, and their separation from cell A in a conventional system determines the carrier to interference ratio (C/I), all other parameters being equal.
However, unlike a standard mobile customer-based cellular telephone system, where subscriber transceivers are relatively low power and employ (gain-limited) omnidirectional antennas, the geographical locations of the radios of a wireless local loop are fixed, which allows the WLL customer radio site to employ an antenna having a focussed directivity pattern (namely one providing gain toward the base station of that cell). While this facilitates communications for a geographically large WLL cell, there still remains the fact that frequency channels are a precious resource, and their reallocation is governed by the allowable carrier to interference ratio.
In accordance with the present invention, advantage is taken of the fixed location of each subscriber""s radio, and the ability to independently configure each subscriber""s antenna, so as to incorporate communication signalling or antenna polarization diversity into the cellular layout of the WLL network, and thereby provide an additional layer of interference rejection. Such a cross or orthogonal polarization assignment scheme, in combination with frequency reuse allocation, provides a substantial increase in the C/I ratio and thereby allows an expansion of the reuse of frequency channels.
For this purpose, in a non-limiting example of the polarization diversity mechanism of the present invention, cells of a given row of a cell cluster are assigned a first communication signal or antenna polarization, such as vertical or right hand circular polarization, as non-limiting examples, while cells of adjacent rows are assigned a second communication signal polarization, such as horizontal or left hand circular polarization, as non-limiting examples, the second polarization being xe2x80x98orthogonalxe2x80x99 to the first polarization for maximum polarization isolation. All the cells of any given row employ the same polarization. Such a cross polarization assignment scheme allows cells in adjacent rows to employ identical channels without exceeding the established carrier to interference ratio (C/I). For any given cell, only two adjacent cells have the same polarization as the cell of interest, while the remaining four adjacent cells have the opposite polarization, so that cross polarization isolation is maximized.
When such an interleaved row cross or orthogonal polarization scheme is applied to a hexagonal cluster diagram employing a frequency reuse factor of four, an increase in interference rejection is provided by a cross polarization isolation of 10 dB between a given horizontally polarized base station antenna, and vertically polarized antennas employed in cells of rows on either side of the row containing the base station of interest.
Actual orthogonal polarization isolation will vary as a function of the RF channel and may be maximally established at the time the WLL network is initially configured. For such a cell cluster having a frequency reuse allocation of four, if orthogonal or cross polarization isolation were infinite, the resultant C/I would provide a 4.8 dB improvement over a network not using polarization diversity.
If the orthogonal polarization isolation is only 10 dB, however, the C/I improvement still enjoys a substantial improvement of 4.0 dB. Additional improvement can be obtained by installing subscriber antenna with directional gain. In general, a base station cannot employ directional antennas, but instead uses a single omnidirectional antenna having a polarization dictated by the designed polarization diversity scheme.